The mystery of how one of Britain's longest-serving and best-placed spies smuggled scientific documents about Hitler's nuclear weapons programme out of Nazi Germany are concealed, it is alleged, within the secret service's archives.

Cherie Booth QC, the former prime minister's wife, appeared in court yesterday in an attempt to rescue the reputation of Paul Rosbaud - reputedly the longest-serving and best-placed spy working for Britain during the second world war - from oblivion.

In a test case that could force the service to disclose more of its archives, Ms Booth argued that the heroic role of Rosbaud, who died in 1963, should be widely appreciated, and accused the intelligence service of resisting the culture of open government.

Yesterday's hearing, at the offices of an employment tribunal in central London, was the culmination of years of campaigning by Vincent Frank-Steiner, the nephew of the Austrian-born secret agent who was a trained physicist.

Although hundreds of MI5 and GCHQ files have been released to the National Archives at Kew, none has been intentionally handed over by the Secret Intelligence Service (SIS) - commonly known as MI6.

Ms Booth was representing the surviving Rosbaud family in their application to the investigatory powers tribunal, a body established in 2000 to inquire into complaints of alleged misconduct by the intelligence services - MI5, MI6 and GCHQ.

Three years ago documents were published detailing the extraordinary acts of espionage and bravery carried out by Major Frank Foley, the MI6 station chief in Berlin in the run-up to the war.

Foley used his official position as passport control officer in the embassy to save thousands of Jews from the death camps. He helped Paul Rosbaud send his Jewish wife, Hilde, and their only daughter, Angela, to the safety of the UK. But Rosbaud, who worked as a scientific journalist, insisted on remaining in Germany to fight Hitler's regime from within.

Born in Graz in 1896, he served in the Austrian army during the first world war. Rosbaud's experience of being captured by British forces, and his appreciation of their civility, created an enduring impression.

After completing a doctoral thesis in Germany, his skill and personal charm enabled him to gain access to Germany's leading physicists - including those attempting to build an atomic bomb.

Foley appreciated the privileged position he occupied within the Nazi scientific community and recruited him as a British agent. Codenamed Griffin, he soon began providing London with detailed information on Hitler's weapons programme. One of his first coups, in January 1939, was to publish in his scientific journal, Naturwissenschaften, work on nuclear fission by the physicist Otto Hahn.

Its publication alerted the international physics community and encouraged Albert Einstein to write to President Roosevelt warning that the Germans had begun a nuclear programme.

Rosbaud is also believed to have supplied British intelligence with information about V2 rocket bombs and confirmation that Nazi efforts to construct an atom bomb had been unsuccessful. Much of his information was smuggled out through Norwegian and French resistance networks. Coded messages were sent using numerical references to pages, lines and words in commonly available textbooks.

Three weeks after the end of the war he was brought back to Britain and began a new life: founding Pergamon Press, scientific publishers, with Robert Maxwell.

A biography of Rosbaud was published in 1986 but it raised fresh questions. How, for example, could he have escaped the attentions of the Gestapo for so long? And why did the British apparently not inform the Americans that the Nazis' nuclear programme had failed?

A film about Major Foley's exploits is due to go into production next year. There is one fleeting reference to Rosbaud in an official history of MI6 which mentions a "well-placed writer for a German scientific journal who was in touch with the SIS from spring 1942". The true extent of his espionage, however, remains a mystery.

A letter from MI6 responding to Dr Frank-Steiner's initial request, said: "It is not SIS practice to confirm or deny whether a person who is alleged to have been an agent of SIS was in fact an agent, as such a practice would be damaging to the work of SIS."

Ms Booth described the claim that Britain's foreign relations would be damaged by disclosure as "fanciful". She said: "Parliament intended there should be scrutiny. The way [MI6 is] approaching this case shows that have not adjusted to the new world where they are accountable to third parties for their decisions. SIS has disclosed its 19th-century archives. So there's clearly some flexibility. We do not accept that rules that are applicable to the government are not applicable to [MI6]."

The court heard that a 1992 statement from the lord chancellor appeared to impose a blanket ban on release of the service's files.

Jonathan Crow QC, for MI6, said there "had to be necessity to make the disclosure" of files in the national interest, "otherwise it would turn SIS into an information bureau".

The release of 19th-century archives, he said, had been permitted because they predated the formation of MI6 in 1909.